I have found that using the new slide themes in Powerpoint 08 is a major contributing factor to the lag in performance. I was experiencing the same thing (i.e., 1-2 second typing lag, and 3-4 second selection lag when clicking through slides).
When I just started creating my slides in a blank format (no color/graphic templates), the lag was gone. You could do this and then apply the templates when you are done if you really want spiffy slides and not deal with the annoying slowing. I know this isn't optimal but it is something.
p.s. I am working on a 2.2ghz MBP with 4 gb of RAM so I know it isn't my machine...
>> Stay informed about: PowerPoint 08 is incredibly SLOW